The present invention is directed to a fail-safe relay system for use in infrared security barriers and other devices which must operate in a fail-safe manner. In an Underwriters' Laboratories (UL) publication entitled "Subject 491, OUTLINE OF THE PROPOSED INVESTIGATION FOR PRESS CONTROLS (QUKU, QUQQ)", February 1975 at paragraph 34, the Operation and Component Failure Requirements for Machine Press Controls are enumerated. A fail-safe operation of the press control under component failure conditions as specified in paragraph 5.3-5.9 include the following conditions with respect to relays: relays with mechanical malfunction (blocked open or closed); relay coils under open conditions; and the relay contacts under permanently weld-closed conditions. All of these conditions must give rise to fail-safe operation of the press control apparatus; that is, either the safe shut-down of the press or continuation of the press control apparatus in an unimpaired manner.
Safe-start operation is also required; that is, if either pair of relay contacts remain closed after shut-down, the interconnected machine cannot be restarted even if no obstruction exists in an area to be guarded against human obstruction.
The above criterion for fail-safe operation has posed a problem in the security barrier field since fail-safe operation of the apparatus, such as an infrared security barrier, must be obtained even if relay contacts of the apparatus are welded closed. To meet this fail-safe criterion as well as the others mentioned above, the present invention incorporates two relays which must both be energized (that is; their relay contacts closed) in order for the interconnected device (such as a machine press) to be activated. This relay system incorporates circuitry which receives pulse information indicating the presence or absence of an obstruction in a guarded area so as to cause the relay contacts to open if an obstruction is sensed. However, after the obstruction is removed and re-start of the interconnected device is attempted, such reactivation can only occur if both pairs of relay contacts are open. In a situation where one of the relay contacts remains closed after sensing an obstruction in the guarded area, reactivation of the interconnected device is prevented. This satisfies UL's standard that any one relay can fail at a given time, but if there is a relay failure, safe-start operation requires preventing the interconnected machines from restarting until the relay is operating normally.
In order to obtain these objectives, the relay system incorporates circuitry which allows reactivation of the relay coils only if no obstruction exists and if both relay contacts are in the open position. Failure of either of these prerequisites to exist prevents reactivation of the interconnected device.
Fail-safe circuitry under the above UL standard requires that any one circuit component failure cannot cause unsafe operation of the interconnected machine. The failure modes for circit components in addition to relays are:
(1) for carbon resistors: open circuit or short circuit, PA0 (2) for wirewound resistors: open circuit, PA0 (3) for capacitors: open circuit or short circuit, PA0 (4) for semiconductor devices: open circuit or short circuit being any two junctions, PA0 (5) for circuit breakers: open circuit or failure to open under over-load or short circuit conditions, PA0 (6) for fuses: open circuit, PA0 (7) for transformers: open circuit or short circuit for any winding.
The present fail-safe relay system is designed to operate in a fail-safe manner for any circuit component failure.
Table 1 lists prior art references in the relay system art.
TABLE 1 ______________________________________ U.S. Pat. No. Inventor Date of Issue ______________________________________ 3,412,292 Forbes 1968 3,660,731 Darrow 1972 3,704,396 McDonald 1972 3,761,738 Kay et al 1973 3,858,095 Friemann et al 1974 3,936,828 Siegal 1974 3,953,770 Hiyashi 1976 4,075,961 Harris 1978 ______________________________________
All of the listed patents disclose relay systems with either means for keeping relays energized in response to a pulse stream or means for controlling machine operation in response to detection of an individual in proximity to a detector. Fail-safe features are disclosed as enumerated below, but none of these references alone or in combination with each other disclose or suggest the circuitry of the present invention, and specifically, none disclose or suggest safe-start prevention of reactivation of the interconnected machine or device under situations of relay contact permanent closing or welding.
Specifically, U.S. Pat. No. 3,412,292, Forbes, discloses a dual relay system for liquid flow control systems which employs proximity switches to control the liquid-flow controllers. The specific embodiment shown in FIG. 3 of Forbes utilizes two relays but dies not disclose the failsafe circuitry of the present invention and specifically does not disclose circuitry for preventing the interconnected device from being reactivated (coming back on line) after a shut-down if the relay contacts remain closed due to relay component failure.
U.S. Pat. No. 3,660,731, Darrow, discloses a fail-safe pulsating peak detection circuit which utilizes current through a relay that is proportionate to the peak values of incoming pulses. This is then used by a full-wave rectifier bridge to energizes a relay coil. This type of circuitry is unlike the fail-safe relay system of the present invention including circuitry for preventing reactivation of an interconnected device when either pair of relay contacts remains in the closed position.
U.S. Pat. No. 3,704,396, Macdonald, discloses a safety device for use on machines. It discloses the use of a plurality of light sources and a corresponding plurality of photoresponsive devices in which each light source is powered by a modulated electric supply and is aimed at the corresponding photo responsive device. The output of each photo responsive device is reshaped and compared with the input of the light source so that any mismatch causes a machine operation to stop. Although multiple light sources and receivers are used in a manner which can be used to generate the pulses for use with the present failsafe relay system, this reference does not disclose or suggest use of dual relays for insuring a fail-safe relay operation.
U.S. Pat. No. 3,761,738, Kay et al, discloses a remote control electrical circuit. Other than the use of a triac type thyristor in the circuitry, this reference does not disclose or suggest the failsafe relay system of the present invention.
U.S. Pat. No. 3,858,095, Friemann et al discloses a protective circuit arrangement for band cutter machines so as to immediately stop the band cutter when it is touched. A fail-safe relay system is not disclosed in this reference.
U.S. Pat. No. 3,936,828, Siegel, discloses an electronic protection and sensing apparatus including an antenna positioned about equipment to be protected and coupled to a resistor-capacitor bridge to which an oscillator is connected. Although this reference discloses apparatus for use in protecting operators of machinery as do infrared security barriers for which the present fail-safe relay system may be employed, this reference does not disclose or suggest the fail-safe relay system of the present invention.
U.S. Pat. No. 3,953,770, Hayashi, also is directed to safety equipment for machinery including presses and shearing machines which utilizes generation of a high frequency electric field for sensing the presence of an operator's hand so as to insure that the operator's hand does not enter a protected area. This reference does not disclose or suggest the failsafe relay system of the present invention which can be used in conjunction with such safety equipment.
Finally, U.S. Pat. No. 4,075,961, Harris, discloses a proximity detector device which utilizes radio frequency signal detection for energizing circuitry which in turn controls a relay for de-energizing an interconnected achine. The relay is not of a failsafe nature as presented in the present invention.
Consequently, although a number of prior art references are directed to relay systems and devices used for protecting operators from hazardous regions of machinery (for which such fail-safe relay systems can be utilized), none of these references alone or in combination with each other disclose or suggest the features of the failsafe relay system according to the present invention, including fail-safe relays and fail-safe electronic circuitry.